The present invention relates generally to a sewing machine capable of sewing buttonholes, specifically eyelet-end buttonholes and other patterns of buttonholes having a circular stitching part. More particularly, the invention is concerned with a sewing machine which permits formation of such buttonholes having a neat, beautiful appearance.
In the art of sewing a predetermined buttonhole having a circular stitching part, a buttonhole sewing machine has been proposed which has a data generator generating bight and feed data representative of each stitch position in timed relation with reciprocation with a needle, and which is operable in one of two modes or conditions selectable through manually operated means: an automatic condition wherein the lateral bight position of the needle and the incremental feed displacement are automatically controlled by the respective bight and feed data from the generator; and a manual condition wherein at least one of the bight position and the feed increment is manually adjusted according to adjustment signals which are generated from a data adjusting circuit to change the bight data and/or feed data from those stored in the generator. In this type of sewing machine, a predetermined buttonhole is sewn exactly from stored stitch data, when in the automatic condition, such that its circular stitching part and side stitching parts connected to the circular stitching part are balanced in size to provide a beautiful appearance, and when the manual condition is established by the manually operated means, the lateral needle jogging and incremental workpiece feeding amounts are manually adjusted to desired values whereby the buttonhole is formed without a trouble of excessively high density of stitches, i.e., stitches which are too close together, or without other troubles which could occur due to changes in workpiece thickness and material or variation in thread thickness.
When the incremental feed of the workpiece is adjusted, the size of a circular stitching part is changed according to adjusted workpiece feed increment while those of side stitching parts are determined by the size of a button. This may cause the formed buttonhole pattern to be unbalanced in size between the circular and side stitching parts, spoiling a neat appearance of the buttonhole dictated by the stored stitch data. When the lateral jogging amount of the needle is adjusted, on the other hand, the circular stitching part tends to be compressed lengthwise of the buttonhole because of limitation in maximum amount of the bight adjustment. This also spoils the appearance of the formed buttonhole pattern and degrades the commercial value of a product having the buttonhole. In addition to the above disadvantages, the known buttonhole sewing machine has been found to have another disadvantage that the adjustment of the pattern width (needle jogging amount) to a smaller value causes reduction in inner diameter of the circular stitching part, which reduction may result in increased chances for cutting stitches when the workpiece is cut to form a buttonhole slit.